1. Field of the Invention
Aspects of the present invention relate to an object lens with a high numerical aperture that can be realized at a low cost and an optical pickup device adopting the same.
2. Description of the Related Art
In an optical recording and/or reproducing apparatus which records and/or reproduces information to and/or from an optical disc, which is a type of optical information storage medium that uses light focused by an object lens, the recording capacity of the optical information storage medium is determined by the size of a light spot. The size of a light spot S is determined by the wavelength λ of the light used and the numerical aperture (NA) of an object lens as shown in Equation 1.
                    S        =                              2            π                    ·                      λ            NA                                              Equation        ⁢                                  ⁢                  (          1          )                    
Thus, to reduce the size of a light spot formed on an optical disc for high density recording and/or reproducing to and/or from the optical disc, the use of a short wavelength light source and a high NA object lens are necessary.
A digital versatile disk (DVD) uses light having a wavelength of 650 nm (or 635 nm) and an object lens having an NA of 0.6 (0.65 for a recordable type), as is well known. When the diameter and track pitch of the disk are respectively 120 mm and 0.74 μm, the DVD has a recording capacity of about 4.7 GB or more for a single side. Thus, the DVD is insufficient as a recording medium for recording high definition (HD) level moving picture information. When moving picture information of 135 minutes is recorded in an HD level, a recording capacity of, for example, 23 GB or more for a single side is needed.
To meet the above demand for a high density recording capacity, there is a demand for a high density optical disc (that is, a next generation DVD (hereinafter, referred to as the HD-DVD (high definition-DVD))), which has a narrower track than a DVD and uses a blue light having a wavelength in the range of about 405 nm through 408 nm and an object lens having an NA of 0.6 or more. Also, companies are widely developing the Blu-ray disk (BD) as a next generation high density optical information recording storage medium which uses an object lens having a high NA, such as 0.85. The BD uses a blue light of a short wavelength (in the range of about 405 nm through 408 nm) as a light source, and has a recording capacity of about 20 GB or more.
FIG. 1 shows the structure of a conventional object lens. Referring to FIG. 1, a conventional object lens 10 includes a first surface 12 that is a light incident surface and a second surface 14 that is a light exit surface. The first surface 12 is a spherical or aspherical surface having a large curvature. The second surface 14 is a flat surface or has a relatively small curvature. As a material of the object lens 10, a plastic material is preferred to a glass material in view of factors to be considered for mass production, including yield, processing ability, and life span. However, since the refractivity of plastic is generally lower than that of glass, it is difficult to manufacture the object lens 10 with a high NA in the form of a single unit lens by using plastic.
Specifically, it is difficult to manufacture the object lens 10 having a high NA and using plastic while satisfying an allowance condition of optical aberration due to the following manufacturing limitation. For the object lens 10 to have a short focal distance and a high NA, the lens surfaces 12 and 14 need to have a large curvature so that an inclination angle of the first surface 12 increases. When the inclination angle exceeds an allowance limit, the manufacturing of a mold becomes difficult, and accordingly, a production yield is lowered. Thus, an object lens having a high NA which can be easily manufactured and falls within an assembly allowance is necessary.
In addition, for an object lens used in an optical pickup apparatus which functions with a Blu-ray disk, since a laser diode of a blue wavelength is sensitive to a wavelength change according to the change in temperature, an object lens capable of correcting chromatism according to the wavelength change is necessary.